In conventional semiconductor inspection systems, an ordinary I/O pin electronics circuit includes an I/O common pin 10 which functions as a driver (DR) and comparator (CP) as shown in FIG. 3(a). The DR and CP are connected with a device under test (DUT) to be tested 13 by a cable having a propagation delay time Ta. As is well known in the art, the driver (DR) provides a test signal to the device under test (DUT) and a resultant output signal from the DUT is compared with an expected signal produced by the semiconductor inspection system by the comparator (CP). Also well known in the art, during the test, each DUT is inserted in a DUT socket provided on a test station of the semiconductor inspection system.
FIG. 3(b) is a diagram showing timing for writing and reading operations.
Data R which is read out from the DUT reaches the comparator CP after a propagation delay time Ta. In order to insure that the DUT receives the write data W immediately after the completion of this reading operation, write data W is output from the DR at a time Ta before the writing operation begins in the DUT. Thus, the data W reaches the DUT just as the writing operation begins at the DUT. However, the data W outputted from the driver DR reaches the comparator CP without any delay. Thus, as soon as the data W is output by the driver DR, the data R, which is being read from the DUT, and data W, which is outputted from the DR, interfere (i.e., combine) at the comparator CP. The time during which the interference takes place is twice as long as Ta, so that proper comparison measurements cannot be performed by the comparator CP during this period of time. This period of time is called I/O dead band 20, and is determined by the propagation delay time Ta between the DR and CP and DUT.
In order to measure the propagation delay time Ta, an open circuit is formed at the DUT side as shown in FIG. 4(a). A waveform output from the driver DR is transmitted to the open end of the cable. Thus, the reflection of the transmitted wave returns through the same cable and is detected by the comparator CP. This waveform is shown in FIG. 4(b), and the reflection time can be measured from the waveform by an appropriate measuring instrument such as an oscilloscope, a time interval counter or a time domain reflectometer. Namely, the propagation delay time Ta of the cable is one-half of the time it takes to detect the reflected wave after transmission of the original wave through the cable.
In inspecting a high speed device where the I/O dead band causes a problem, a separate I/O test is performed, wherein the transmission path from the DUT to the CP is separated from the path between the DUT and the DR (see FIG. 5(a)). Due to the split cable configuration of FIG. 5(a), interference will not occur between the data R which is read out from the DUT and data W output from the DR at the comparator CP. Therefore, the proper comparison measurement can be performed by the CP without a dead time. Of course, it will be noted that the waveforms of the read-out data R and output data W interfere at the output of the DR. The collision of these waves will not cause problems because the waves only pass through each other without modulating one another. Moreover, since the waveform from the DUT ends at the DR end, it will not affect the CP end.
If it is desirable to measure the propagation delay time of the cables in the split cable configuration of FIG. 5(a), then the point VT in FIG. 5(a) is placed in a high impedance state (i.e., an open circuit). A signal is transmitted and reflected as before so that the total delay time Tb+Tc can be measured from the DR to the CP through the DUT. However, the delay time Tb from the DR to the DUT and the delay time Tc from the DUT to the CP cannot be separately measured using this method.